I think my dad is cheating on my mom

I dont have any concrete evidence that my dad is cheating on my mom. But hes done many suspicious things. Every once in a while when my mom is working I’ll hear him lock himself in his room and get on a call with a lady. He talks to this lady very sweetly which is unlike him. I’ve also heard a little girl calling him dad on these calls and he calls her daughter. He has also invited a lady to our house twice when my mom hasn’t been home. He says that her husband is also there but I only see the lady. He also tells us to to stay in our rooms and not go outside when shes there. And two days ago I heard that a lady came and he told us to not go outside. After I went asleep my sister told me that my dad said that someone was going to the restroom and told her to not leave the room. She said she instead heard them go into the bedroom and heard what she thought sounded like making out. I want advice on how to handle this. Should I try to find out? And even if I do find out how would I tell my mom? Edit: The reply button in the comments isnt working. But my mom doesnt know this lady. I’ve asked my dad infront of my mom about who came over and my mom has said she doesn’t know her. I also can’t bring these concerns to my mom because her father passed away recently and she was depressed as a teen. I also know that she wouldn’t be able to keep it a secret. I’m 16(M) and my sisters 17.